1. Field of the Invention
The present invention relates to a raster scanner for scanning a photosensitizer by moving a beam from a beam generating means along the main scanning direction of the photosensitizer and more particularly to improvements of the raster scanner in which a mechanism for adjusting length of a beam path between the beam generating means and the photosensitizer is provided, to a mirror supporting structure which is effective in a type of raster scanner that contains a cylindrical mirror in its beam path and to a method for adjusting angle of the mirror using the mirror supporting structure.
2. Description of the Related Art
A laser scanner used for laser printers and others is a typical raster scanner. As shown in FIGS. 20a and 20b, it is comprised of various parts such as a semiconductor laser 201 for irradiating beam Bm in accordance to image signals, a polygon mirror 202 for directing the beam Bm from the semiconductor laser 201 across a certain scanning range, an imaging lens 204 for imaging adequately the beam Bm from the polygon mirror 202 along the main scan line on a photosensitive drum 203 and a reflecting mirror 205 for guiding the beam Bm directed by the polygon mirror 202 to the photosensitive drum 203 side. In order to keep good precision in scanned position on the photosensitive drum 203, the aforementioned various parts are positioned accurately through intermediary of an optical sub-frame 206 or directly on an optical frame (not shown).
However, dispersions of precision are caused among those parts when they are manufactured and when they are mounted, so that various adjustments are required to absorb those dispersions of precision. One of the adjustments is adjustment of length of optical path, by which a distance from the polygon mirror 202 to the photosensitive drum 203 or that from the imaging lens 204 to the photosensitive drum 203 is adjusted to adjust an exposing width (actual scan range in the scan direction) to a specified time.
By the way, as a prior art optical path adjustment method, there is, for example, a type in which the reflecting mirror 205 is supported by variably adjustable three supporting points as a supporting mechanism of the reflecting mirror 205. In that type, position of the reflecting surface of the reflecting mirror 205 is moved in the optical axis direction to variably adjust the length of the optical path by variably adjusting position of each supporting point. When quality of output image of a laser printer whose optical path was adjusted by this type of optical path adjusting method was examined, however, a technological problem was found that unbalance is apt to be caused in the image width in the main scan direction at the both ends of the image.
On the other hand, as a prior art method for adjusting angle of a reflecting mirror, Japanese Patent Laid-open No. 63-55520, for example, is disclosed. This is characterized in that, as shown in FIG. 21, a rotary supporting plate 212 is rotatably mounted on one side of a pair of securing plates 210 and 211, a hole 213 for inserting the reflecting mirror 205 is opened on the rotary supporting plate 212, supporting points 214 and 215 for supporting the reflecting surface of the reflecting mirror 205 at two points and a supporting point 216 for supporting the bottom side surface of the reflecting mirror 205 are created on the edge portion of the mirror inserting hole 213 and the reflecting mirror 205 is urged to each supporting point by a plate spring 217. A mirror inserting hole 218 is opened also on the other securing plate 211, a supporting point 219 for supporting the reflecting surface of the reflecting mirror 205 at one point and a supporting point 220 for supporting the bottom side surface of the reflecting mirror 205 are created on the edge portion of the mirror inserting hole 218 and the reflecting mirror 205 is urged to each supporting point by a plate spring 221.
According to this type of method, when the one rotary supporting plate 212 is rotated appropriately, one end of the reflecting mirror 205 rotates following to that and the other end of the reflecting mirror 205 rotates by a certain angle pivoting on the supporting point 218. Thereby the reflecting mirror 205 is adjusted to the certain angle.
By the way, in order to correct an inclination of mirror surface of the polygon mirror 202, a cylindrical mirror, for example, is used as the reflecting mirror 205. When using a cylindrical mirror, however, its generating line has to be kept in parallel, differing from a plane mirror, and normally a measure for preventing the cylindrical mirror from rising up needs to be taken by providing plate springs for pressing the cylindrical mirror downward.
When the angle of the cylindrical mirror was adjusted by this type of method, however, a new technological problem was found that the generating line of the cylindrical mirror inclines, which damages the performance for adjusting the angle of the cylindrical mirror.